Validation in DevForce is done by default when an entity changes -- property level validation when a property changes, and instance-level validation when saved. If the Car object here hasn't changed, it won't be validated by default. You can force validation on an object or one of its properties by calling one of the overloaded VerifierEngine.Execute* methods.

Since you essentially have custom validation needs here, a custom verifier would likely help. With a custom verifier you write the validation logic and determine when the verifier should be triggered. Here's some information from the DevForce Resource Center to get started -- http://drc.ideablade.com/xwiki/bin/view/Documentation/validation-create-custom-verifier.

Navidation Property : Car